A. Technical Field
The present invention relates to methods of stabilizing and/or sealing the core of a molded composite structure and the stabilized and/or sealed core of such structures. In particular, the present invention relates to methods of manufacturing aircraft from such structure.
B. Technology Background
Presently, composite materials (such as fiber-reinforced plastics) are increasingly being used to manufacture aircraft. The manufacture of such aircraft with composite materials involves the fabrication of the major parts of the aircraft, including the fuselage, the wings, the empennage (tail), and the various other components of the aircraft. Typically, a layered stack of composite materials or a “material stack” is combined with a resin, placed on tooling, and cured to form a structure. In some cases, the material stack includes a central member called a core, which is intended to stiffen the material stack and thereby stiffen the structure created using that material stack. The core may include honeycomb structures, foam, or even wood. The core is referred to as a central member because, typically, the core is located between layers of composite materials. It should be understood, however, that core need not be located at the exact center of a material stack.
In one manufacturing method know as Resin Transfer Molding, or “RTM,” a composite structure is fabricated by infusing resin into a closed mold into which a material stack has been placed, and then press curing the structure. An example of this type of manufacturing method is disclosed in U.S. Patent Application Publication No. 2002/0069962, published on Jun. 13, 2002, the disclosure of which is expressly incorporated herein. In another method, a material stack is placed on a mold having only one tool surface. A flexible membrane is then placed over the material stack and mold. Heat and pressure are then applied to the flexible membrane to consolidate and cure the material stack.
The use of core in these manufacturing methods, however, can create several problems. For example, in the RTM process, the injection of resin into the tooling may cause resin to infiltrate into either the cells of the core in honeycomb core, into the open spaces among the small foam cells in closed cell foam core, or into the foam cells themselves in open cell foam core. This results in undesirable weight gain. Therefore, in order to prevent this weight gain, the core is often sealed to prevent the infiltration of resin.
In addition, in the case of honeycomb core, the pressure exerted on the core by the injection of resin or by the use of an autoclave to apply pressure and heat may cause the cells in the core to distort or collapse. This can also result in deformation of the composite structure as a whole. Therefore, in order to prevent this deformation, the honeycomb core often needs to be stabilized.
Typically, a core is sealed using barriers on the outer surfaces of the core. The barriers prevent the intrusion of resin into the core. The barriers are usually constructed of bondable thermoplastic films. Further, in order to stabilize the core in the direction perpendicular to its thickness, a support layer is often placed on the surfaces of the core. The support layer is typically a fibrous material (such as glass, carbon or an organic compound) in woven cloth, chopped, or random strand mat form impregnated with epoxy resins. Both a stabilization layer and a barrier are usually necessary because the stabilization layer is typically porous and will not prevent the intrusion of resin into the core.
Presently, film adhesives are used to bond the support layer to the core and to the barrier. Film adhesives are normally comprised of epoxy resin materials in the form of a thin film, such as NB101A manufactured by Newport Adhesives and Composites of Irvine, Calif. After application of the support layer, the core is subjected to heat to cure the film adhesive, which then bonds the support layer to the core and barrier, thereby, sealing and stabilizing the core.
These methods of sealing and stabilization, however, have certain drawbacks. For example, the film adhesives used in the present method are expensive and may be twice as heavy as needed to perform the required function. For some thin core applications, this added weight approximately doubles the area density of the core and may triple the cost of the core structure. In addition, the stabilizing material is not generally relied on structurally, i.e., not counted upon for load carrying. Finally, this method relies on labor to prepare the surface of the sealed and stabilized core for subsequent bonding. Thus, there is a need for a method of stabilizing and/or sealing a core material without these deficiencies.
Apparatus and methods consistent with the invention provide for a method of stabilizing and/or sealing a core material without adding considerable cost and weight to the structure.